Counterparty Risk (Cash and Cash Equivalents) (Details)	Jun. 30, 2016	Dec. 31, 2015
Bank of New York Mellon Corporation [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	62.00%	13.00%
BlackRock Liquidity Temp Fund [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	36.00%	54.00%
US Bank [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	2.00%	0.00%
J.P. Morgan US Dollar Liquidity Fund [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	0.00%	27.00%
J.P. Morgan US Treasury Plus Premier Fund [Member]
Concentration Risk [Line Items]
Concentration risk percentage, cash and cash equivalents	0.00%	6.00%